The objective of this application is to clarify how a novel cytosolic chaperone complex promotes endoplasmic reticulum (ER) membrane penetration of the non-enveloped virus polyomavirus (Py). To cause infection, Py engages a host glycolipid receptor on the plasma membrane, becomes endocytosed, and traffics to the ER lumen. Here the viral particle undergoes conformational changes to penetrate the ER membrane and reach the cytosol. From the cytosol, the virus transports to the nucleus where transcription and replication of the viral genome ensue, leading to lytic infection or cellular transformation. How Py enters the cytosol from the ER is a crucial yet poorly understood event. Particularly enigmatic are cytosolic events that regulate this ER membrane penetration process. Accordingly this application is focused on elucidating the role of the so-called BHS cytosolic chaperone complex in ejecting Py into the cytosol from the ER.